X-ray photoelectron spectrometry (XPS) which is also known as electron spectroscopy for chemical analysis (ESCA) is a useful tool for providing information about the chemical composition of the elements of solid surfaces and the states of chemical bonding. However, an ordinary XPS instrument can collect average information only from a sample surface area of about 1 cm.sup.2.
In recent years, with the development of new materials and failure analysis, it has been required to get information about local sample surface areas and two-dimensional information regarding sample surfaces. Under these circumstances, various contrivances have been made to impart local analysis function or imaging function to XPS instruments. These contrivances are next described briefly.
(a) Method using a scanning electron beam as an X-ray source (C. T. Hovland, Applied Physics Letters, Vol. 30, No. 6, pp. 274-275 (1977), and J. Cazaux, Ultramicroscopy, 12, pp. 321-332 (1984)): Al or Mg that is used as an X-ray anode is deposited on the rear side of a thin sample. An electron beam is directed to the anode from the side opposite to the sample. At this time, the electron beam is scanned to move the illuminating electron beam spot. X-rays produced at each spot propagate through the anode and impinge on the sample. As a result, photoelectrons are ejected from the sample. The photoelectrons are analyzed by a coaxial cylindrical mirror analyzer (CMA). With this method, an XPS image is obtained at a resolution of about 20 .mu.m.
(b) Method involving projection of a magnetic field (G. Beamson, H. Q. Porter and D. W. Turner, J. Phys. E: Sci. Instrum. Vol. 13, pp. 64-66 (1980), and G. Beamson, H. Q. Porter and D. W. Turner, Nature, Vol. 290, pp. 556-561 (1981)): This method was developed by Beamson and others in 1980. Photoelectrons trapped in a diverging magnetic field formed by a superconducting magnet move along the magnetic flux and follow a synchrotron orbit. The image of the photoelectrons is magnified and projected onto a screen while the information about the positions at which the photoelectrons are ejected is maintained. A spatial resolution of several microns is achieved.
(c) Method of collecting photoelectrons using a spectral crystal (Robert L. Chaney, Surface and Interface Analysis, Vol. 10, pp. 36-47 (1987)): X-rays emitted from an anode are made monochromatic by a spectral crystal and focused onto a sample surface. An illuminated area having a diameter on the order of 100 .mu.m is realized. The local area illuminated with X-rays can be subjected to spectral analysis. Then, the sample stage is scanned by driving the motor to obtain an XPS image.
(d) Method of limiting the field of view (D. J. Keast and K. S. Downing, Surface and Interface Analysis, Vol. 3, pp. 99-101 (1981), and K. Yates and R. H. West, Surface and Interface Analysis, Vol. 5. pp. 217-221 (1983)): A wide area of a sample surface is uniformly illuminated with X-rays. Photoelectrons are ejected simultaneously from the illuminated area. The input lens of the analyzer is equipped with an aperture that limits the field of view, to detect only the photoelectrons emitted from a local area. This permits spectral analysis of the local area.
(e) Method of magnifying image at plural stages and projecting it, using an intermediate energy analyzer (U.S. Pat. No. 4,758,723 and P. Coxon, J. Krizek, M. Humpherson, and I. R. M. Wardell, Journal of Electron Microscopy and related Phenomena 52, pp. 821-836 (1990)): A wide area of a sample surface is uniformly illuminated with X-rays. Photoelectrons are ejected from various locations on the sample surface. From these photoelectrons, a magnified image of the sample is created by a lens system. The magnified image is brought into the front focal point of a lens located before the entrance of a coaxial spherical sector electron energy analyzer. Thus, the electron beam impinging on the analyzer is collimated. The beam is again focused after leaving the analyzer. The resulting image is created by monoenergetic photoelectrons. The XPS image is produced at a resolution of about 10 .mu.m.
(f) Pre-lens scanning system (M. P. Seah and G. C. Smith, Surface and Interface Analysis, Vol. 11, pp. 69-79 (1988)): As schematically shown in FIG. 6, a sample surface is uniformly illuminated with X-rays emitted from an X-ray source. Photoelectrons ejected from various locations on the sample surface are analyzed by a system comprising two pairs of deflection plates, a lens, an input aperture, and a coaxial spherical sector electron energy analyzer. A scanning voltage is applied between the two pairs of deflection plates to deflect the incident photoelectrons. Only the photoelectrons ejected from a desired location on the sample surface can be detected. This method enables local analysis of an area about 100 .mu.m in diameter. Also, an XPS image with a spatial resolution of about 100 .mu.m is derived.
We now discuss the methods described above. The method (a) using a scanning electron beam as an X-ray source was attempted in the earliest years. This method was worth noticing only in that it opened up the techniques for local XPS analysis and for imaging XPS systems. However, it is necessary to deposit a sample on the X-ray target surface. Therefore, the sample cannot be analyzed unless it can be deposited. Furthermore, the spatial resolution and the intensities of photoelectrons are affected by the thickness of the deposited film. Hence, it cannot be said that this method is a generally accepted technique.
In principle, the methods (b) and (e) are the best methods of effecting XPS imaging, because photoelectrons ejected from the analyzed area are simultaneously detected, whereby an XPS image can be obtained in a short time. Nonetheless, the method (b) involving projection of a magnetic field needs strong magnetic force as produced by a superconducting magnet. Therefore, complex steps are necessary to fabricate the apparatus, thus increasing the cost. The method (e) necessitates several stages of lenses and so the apparatus is complex. Furthermore, it is difficult to operate the apparatus. Additionally, the apparatus is expensive.
The method (c) of collecting photoelectrons, using a spectral crystal provides good performance in local analysis, but it is necessary to scan the stage in two dimensions, for XPS imaging. This two-dimensional scan takes long times to create an XPS image because the amount of signal is small. The method (d) of limiting the field of view is evaluated essentially as highly as the method (c). However, the method (d) is inferior in intensity to the method (c).
The pre-lens scanning system (f) differs from the methods (c) and (d) in that XPS imaging is enabled without the need to scan the stage. That is, the method (f) is simpler than the methods (c) and (d). However, this method takes a long time to perform, because small amounts of photoelectrons are successively accepted by two-dimensional scan. Further, whenever photoelectrons of other given energy should be obtained, the voltage applied to the pairs of deflection plates must be adjusted so that photoelectrons of the energy of interest pass through the input aperture.